An airbag device is installed on the central part of a steering device 50P, for example, as shown in FIG. 16, and a member 40P for display is provided on the central part of the airbag device, and on the outer circumference thereof, an airbag cover 16P is provided, having tear-lines on the back side that can be split and torn into a plurality of cover pieces 16Pb when the airbag 16P undergoes inflation and expansion, containing the folded airbag and an inflator therein (see Patent Document 1).
FIG. 17 and FIG. 18 are sectional views showing another airbag device provided with the same structure as that shown in FIG. 16. A diffuser 32R provided with gas exhaust holes 30Ra for exhausting gas is disposed on an inflator 30R of this airbag device, and an inflator attachment opening of an airbag 10R is secured to a position denoted by Q, while a tether belt 11Ra for constraining an extension length of the airbag 10R is provided on the central part of the airbag 10R, and the end of the tether belt 11Ra is secured by a fixture member provided on the head of the inflator.
As is evident from FIG. 17, the diffuser 32R provided with a plurality of gas exhaust holes 32Ra is disposed between the inflator 30R and the folded and contained airbag 10R. At the time of actuation of the inflator 30R, a gas generated from the inflator 30R passes through the gas exhaust holes 32Ra and applies a gas pressure directly to the central part of the folded and contained airbag 10R (see Patent Document 2).
With the structure of this airbag device, the gas pressure is concentrated in the central part of the folded airbag 10R at the time of actuation of the inflator 30R so that a state of the folded airbag 10R is collapsed and the airbag portion above the gas exhaust holes 32Ra is pushed out exponentially in the direction of expansion of the airbag, to thereby enable a tear-line 16Ra provided on the back side of the airbag cover 16R to be ruptured, causing the airbag portions to burst out largely toward a driver seat.
Due to an inertia force of the airbag portions as burst out, a remaining portion of the airbag bursts out at once toward the driver seat. This bursting-out is said to be as “a punching phenomenon” and there is a possibility that the maximum bursting-out velocity reaches 300 km/h, thereby inflicting harm on an occupant. According to the invention disclosed in Patent Document 2, it seems that an opening of the airbag cover 16R for expansion of the airbag 10R at the time of rupture of the airbag cover 16R is rendered small as a countermeasure against such a punching phenomenon so that the airbag 10R is not burst out at once, resulting in reducing damage caused by the punching phenomenon. However, if the opening is rendered small, when the folded and contained airbag 10R is pushed out at once toward the opening, the airbag 10R is clogged at the opening of the airbag cover so that the airbag portion positioned in the vicinity of the opening is difficult to protrude toward the driver seat due to a gas pressure to be applied from behind.
In this state, since the airbag 10R is difficult to protrude, there is a possibility that the gas pressure inside the airbag 10R further increases and the airbag cover 16R itself undergoes inflation, so that the airbag cover 16R increases the inflation velocity while forcibly expanding its opening so that the airbag 10R bursts out through the opening of the airbag cover 16R. In addition to that, in cases where only the portion which is slipped out of the opening in the initial stage of expansion of the airbag 10R first busts out to undergo inflation and expansion partially, the direction of expansion of the airbag is changed in whole to cause the direction of expansion unstable, thereby deteriorating performance of the airbag device to protect the occupant.
Next, there is described still another conventional airbag device. FIG. 19 is a traverse sectional view of this airbag device. As shown in the figure, an airbag 10L is folded and contained between an airbag cover 16L and an inflator 30L. A tear-line 16La in the shape substantially resembling the letter H is provided on the back side of a front face wall of the airbag cover 16L, and an inflator attachment opening of the airbag 10L is secured to a flange 30Lb provided on the outer circumference of the inflator 30L (see Patent Document 3).
According to this airbag device, a gas pressure is concentrated in the whole back portion of the folded airbag 10L at the time of actuation of the inflator 30L, and concurrently the lump of the airbag 10L above the gas exhaust holes 30La is pushed out exponentially in the direction of expansion of the airbag, to thereby enable the tear-line 16La in the shape substantially resembling the letter H provided on the back side of the airbag cover 16La to be ruptured so that these airbag portions largely burst out toward the driver seat. Although it is possible to control a quick bursting-out of the airbag in a specific folding manner, it requires a folding manner wherein the airbag is normally difficult to undergoes expansion in order to control a quick bursting-out of the airbag, causing a problem in that expansion time until the airbag is fully expanded is delayed. On the other hand, if the airbag is folded not to delay the expansion time, there occurs a punching phenomenon described as above.
FIG. 20 is a view showing each state of inflation of the airbag with respect to time elapsed from the initial expansion of the airbag of a conventional airbag device. A dummy is postured in a seating position close to the airbag (this seating position is said to be out of normal seating position i.e. Out of Position, hereinafter referred to as “OOP state”). The airbag is formed in the shape of a bag that is expandable into a flat spherical shape (an ellipsoidal shape) by preparing a stitched part P by stitching respective outer circumferential edges of two pieces of cloth substantially circular in shape. Accordingly, the stitched part P is positioned in the center of the side face of the flat spherical airbag at the time of completion of expansion.
As shown in FIG. 20A to 20D, when the airbag undergoes inflation in the manner that the stitched part P thereof rests against the neck of the dummy, the portion of the airbag rested against the neck of the dummy is controlled in inflation immediately before the airbag is completely inflated, so that the controlled portion, particularly the portion that becomes maximum in airbag diameter, undergoes inflation exponentially in the completion stage of inflation of the airbag, leading to the possibility of inflicting harm on the neck. A phenomenon which occurs when the gas generated from the inflator increases the inner pressure of the airbag exponentially just before or at a moment of the completion of inflation (a phenomenon in which an occupant is repelled by the airbag which is caused by the increase of the inner pressure of the airbag exponentially from a loosened state) is said to be a so-called “a membrane phenomenon”.
Meanwhile, as the method of folding the airbag, there are known (1) a method of folding like bellows the airbag, (2) a method of folding the airbag in the shape of a roll, (3) a method of folding like bellows the airbag in the direction of extension of the airbag sterically (disclosed in Patent Document 2), and so forth. Although Patent Document 3 does not disclose with which method the airbag is folded, the punching phenomenon will occur in any folding method except the specific folding manner disclosed as above. However, there was no specific means to prevent occurrence of the membrane phenomenon in the OOP state disclosed as above except the regulation of opening manner of the airbag cover.                Patent Document 1: U.S. Pat. No. 6,739,620-B1        Patent Document 2: US-2001/42978-A1        Patent Document 3: JP-3-38254-U        